1. Field of the Invention
The present invention relates to a circuit board and electronic equipment using the same and particularly, to a circuit board on which insertion-type electrical parts are mounted by using lead-free solder, and electronic equipment using the same.
2. Description of the Related Art
The construction of a conventional board circuit will be described in detail with reference to FIGS. 1 to 6.
FIG. 1 is a perspective view showing a soldered portion of a circuit board on which electrical parts are mounted by using solder, FIG. 2 is a cross-sectional view taken along a line b-bxe2x80x2 of FIG. 1 and FIG. 3 is a cross-sectional view that schematically shows an occurrence state of land exfoliation. FIGS. 4 to 6 are diagrams showing occurrence of land exfoliation confirmed in a verification test, wherein FIG. 4 is a cross-sectional photograph of A portion (right-and-left inversion) of FIG. 3, FIG. 5 is a cross-sectional photograph of B portion of FIG. 3 and FIG. 6 is an enlarged photograph of C portion of FIG. 5.
Conventional circuit board 11 is formed as follows. That is, as shown in FIGS. 1 and 2, a paper base, a glass base, a polyester fiber base or the like is impregnated with epoxy resin, phenol resin or the like to form an insulating sheet, and then copper foil is attached onto the insulating sheet by a pressuring and heating treatment to form a copper-coated laminate board. Thereafter, a through hole is formed at a desired place of the board, catalyst is supplied to the side surface of the through hole, and then electroless copper plating is applied to the side surface supplied with the catalyst to carry out rough plating. Thereafter, copper electroplating is applied onto the roughly-plated side surface of the through hole to form a conductor (conductive film). The conductor thus achieved and the copper film on the copper-coated laminate board are connected to each other to form through hole 4. Thereafter, the conductive film of copper on the copper-coated laminate board is etched to form land 2.
Finally, solder resist 5 is printed and coated onto the board so that tin-lead solder 12 is not coated onto the portion other than the land 2 on which soldering is conducted, and then exposed to light to form circuit board 11. At this time, the solder resist 5 takes a role of protecting circuits 7 other than the land 2 on which lead 3 is mounted.
Here, the solder resist 5 on the circuit board 11 is printed so as to have an area larger than that of the land 2 and thus so that no solder resist 5 is covered on the land 2. This is because formation of fillet 12a of tin-lead solder 12 would be disturbed if the solder resist 5 is covered on the land 2 in the soldering process using tin-lead eutectic solder (Sn of 63 weight %, Pb of the remaining weight %) which is most frequently used in a soldering joint process for electronic equipment.
Following the recent high-density mounting design, the land 2 is designed to be as small as possible within the permissible range that the minimum joint strength can be kept. Electronic equipment manufactured by using such a conventional circuit board 11 has no critical problem in reliability because tin-lead eutectic solder takes a role of stress-relaxing thermal expansion mismatch which is caused by joint of foreign matters.
However, environmental pollution of lead has been recently more remarkable because people pay more attention to the environmental pollution, and thus shift to lead-free solder containing no lead has been promoted. The lead-free solder mainly comprises tin, and also contains silver, copper, zinc, bismuth, indium, antimony, nickel, germanium, etc., and it has metal characteristics that are higher in metal tensile strength and creep strength and also less in extension as compared with tin-lead eutectic solder (Sn of 63 weight % and Pb of the remaining weight %, hereinafter referred to as xe2x80x9cPb-63Snxe2x80x9d) which is most frequently used for the solder joint of electronic equipment at present. Therefore, the stress relaxation more hardly occurs at the soldered portion as compared with lead solder. Further, the melting temperature of lead-free solder is equal at 190xc2x0 C. to 230xc2x0 C. (190 to 230 degrees Celsius) which is higher than that (183xc2x0 C.) of tin-lead eutectic solder.
As the lead-free solder are mainly used tin-zinc-based solder (materials that contain a tin-zinc eutectic composition (Sn-9.0 wt % and Znwp) as a main material and are improved in characteristics by varying the amount of zinc or adding other elements are generally referred to as xe2x80x9ctin-zinc-based solderxe2x80x9d, and Sn-8.0Zn-3.0Bi is known as a representative material), tin-copper-based solder (representatively, Sn-0.7Cu-0.3Ag), tin-silver-based solder (representatively, Sn-3.0Ag-0.5Cu) or the like. The tin-zinc-based solder has a merit that the melting point thereof is equal to a low value (about 190xc2x0 C.), however, it must be soldered under an inactive atmosphere or under vacuum because it is liable to be oxidized, so that the soldering work efficiency is low.
Further, the tin-copper-based solder and the tin-silver-based solder have little problem in oxidation. However, the tin-copper-based solder has a disadvantage that it has a high melting temperature (about 230xc2x0 C.) and thus land exfoliation is liable to occur.
The tin-silver-based solder has a merit that the melting point thereof is equal to about 220xc2x0 C. lower than that of the tin-copper-based solder and it can be reduced to about 205xc2x0 C. by adding Bi (bismuth). The melting point of the tin-silver-based solder is lowered by increasing the addition amount of bismuth, however, the tin-silver-based solder has a disadvantage that fillet exfoliation occurs if the addition amount of bismuth is increased.
The glass transition temperature of epoxy-based material which is the main material of the circuit board is equal at 125xc2x0 C. to 140xc2x0 C. When lead-free solder is used, the difference in solidification and shrinkage temperature between the epoxy-based material and lead-free solder is larger than when tin-lead solder is used, so that the stress applied to the joint portion of the lead-free solder is larger. It is apparent from the metal characteristics of the lead-free solder as described above that if insertion mounting is carried out on conventional circuit board 11 by using the lead-free solder 6, the land exfoliation which occurs little in the case of the tin-lead solder 12 occurs frequently.
An occurrence example of the land exfoliation will be described in detail with reference to the accompanying drawings.
As shown in FIG. 3, if soldering using lead-free solder 6 is carried out by using the conventional circuit board 11, a large stress is applied to the land 2 due to the difference in solidification and shrinkage temperature between the board material and the lead-free solder 6, and the land 2 is exfoliated from the circuit board 11, so that the land 2 is kept to be floated from the circuit board 11 (see the A portion of FIG. 3 and FIG. 4). At this time, the circuit 7 connected to the land 2 is together lifted up and drawn with the land 2 and thus suffers excessive stress, so that it can be confirmed that the boundary portion between the land end portion 2a and circuit 7 is greatly deformed and broken (see the B portion of FIG. 3 and FIGS. 5 and 6). That is, it is found that occurrence of land exfoliation causes occurrence of wire breaking of the circuit 7.
The manufacturing of electronic equipment by using the conventional circuit board 11 in which the land exfoliation occurs as described above remarkably lowers reliability to the electronic equipment.
The present invention has an object to provide a circuit board having high reliability in which neither land exfoliation nor fillet exfoliation occurs even when lead-free solder is used.
Further, the present invention has another object to provide electronic equipment having high reliability by using the above circuit board.
In order to attain the above objects, according to the present invention, there is provided a circuit board having circuit wires on the surface and back surface thereof, comprising lands having through holes through which conductive members of electrical parts are inserted, said through holes being coated with a conductive film on the side surfaces themselves, wherein said lands and conductive members are mounted by using lead-free solder and the width of each land corresponding to the difference between the radius of each land and each through hole is set at 0.40 mm or more.
Further, according to the present invention, there is provided a circuit board having circuit wires on the surface and back surface thereof, comprising lands having through holes through which conductive members of electrical parts are inserted, said through holes being coated with a conductive film on the side surfaces themselves, wherein said lands and conductive members are mounted by using lead-free solder, and the width of each land corresponding to the difference between the radius of each land and each through hole is set such a value that the intimate contact strength of said land is larger than said land exfoliating force.
The circuit board according to the present invention can be applied a circuit board having through holes through which conductive members (for example, leads) of electrical parts are inserted and a conductive film is coated to the side surfaces themselves, that is, a double-sided printed circuit board or a multi-layer printed circuit board.
Further, in the present invention, the lead-free solder preferably contains tin-zinc based solder, tin-silver based solder or tin-copper based solder.
Furthermore, in the present invention, there is provided electronic equipment in which electrical parts are inserted and mounted in the circuit board by using the lead-free solder.
Still furthermore, in the present invention, the lead-free solder contains solder that contains lead as impurities to the order of no occurrence of property-change.
As described above, the melting point of the lead-free solder is higher than that of lead-containing solder, and consequently, the stress-relaxing force of the lead-free solder is low, whereby the land exfoliation occurs frequently. According to the present invention, in the circuit board in which the lands to be soldered are formed around the through holes in which the leads of the electrical parts are inserted and mounted, the difference in radius between each land and each through hole (the width of each land) is set at a predetermined value or more, whereby each land and the circuit board can be attached to each other with large force, so that the exfoliation of the lands can be prevented and electronic equipment having sufficiently high reliability can be manufactured even when the lead-free solder is used.